This invention relates to optoelectronic and microelectronic devices and fabrication methods therefor, and more particularly to semiconductor devices having semiconducting oxide layers, and fabrication methods therefor.
Optoelectronic and microelectronic devices are widely used in consumer and commercial applications. Optoelectronic devices include, but are not limited to, light-emitting diodes, laser diodes, photodetectors, optical modulators and/or broad band light sources. Microelectronic devices include, but are not limited to, transistors such as CMOS transistors, field effect transistors, and/or bipolar transistors, field emitters, high-power devices, and/or other integrated circuits. Optoelectronic devices and microelectronic devices will be referred to herein generically as xe2x80x9celectronic devices.xe2x80x9d
Many microelectronic devices are silicon-based. However, for high-temperature and high-power applications, other materials are being investigated and used for microelectronic devices. Similarly, other materials are being investigated for optoelectronic devices having a wider bandgap that can cover a wider range of the optical spectrum. For example, III-nitrides and their alloys having hexagonal structure are being widely investigated for microelectronic devices as well as optoelectronic devices. Mechanical, optical and electrical properties of III-nitrides are described, for example, in the publication entitled III-Nitrides: Growth, Characterization, and Properties to Jain et al., J. Appl. Phys., Vol. 87, No. 3, Feb. 1, 2000, pp. 965-1006. Hexagonal wurtzite polytypes of III-nitrides (InN, GaN, and AlN) can form continuous solid solutions with direct bandgaps ranging from 1.9 eV (InN) to 3.4 eV (GaN) to 6.2 eV (AlN). Thus, the emission wavelength can be tuned from about 653 nm (red) to about 200 nm (deep ultraviolet). The binding energy of excitons in the nitride system is about 20 meV for gallium nitride. Because the stable phase of III-nitrides and their alloys is hexagonal wurtzite, these materials only may be grown via lattice-matching epitaxy on hexagonal substrates such as 6H-silicon carbide (0001) and zinc oxide (0001).
On more practical substrates, such as hexagonal (0001) sapphire (xcex1-Al2O3), epitaxial growth only may occur via domain-matching epitaxy, where integral multiples of lattice constants on major planes of the film and substrate match across the interface, as described, for example, in U.S. Pat. No. 5,406,123 to Narayan, which discusses the epitaxial growth of titanium nitride films on silicon or gallium arsenide substrates. For example, the domain matching epitaxy on the basal plane after 30xc2x0 or 90xc2x0 rotation [1210]nitride∥[0110]sap involves the matching of 7 planes of the III-nitride films with 6 planes of sapphire. The epitaxial films of III-nitrides and their alloys on sapphire contain a high density of growth and misfit related dislocations which may adversely affect the lifetimes of optical devices, particularly lasers, as described in the publication entitled The Roles of Structural Imperfections in InGaN-Based Blue Light-Emitting Diodes and Laser Diodes to Nakamura, Science, Vol. 281, No. 5379, Aug. 14, 1998, pp. 956-961. Epitaxial growth of hexagonal nitrides may be possible on (111) planes of cubic silicon, as described, for example, in the publication entitled Epitaxial Growth of AlN Thin Films on Silicon (111) Substrates by Pulsed Laser Deposition to Vispute, J. Appl. Phys., Vol. 77, No. 9, May 1, 1995, pp. 4724-4728. However, this may not be a practical plane as most silicon microelectronic devices are fabricated on (100) silicon.
As an alternative to III-nitrides, zinc oxide having an hexagonal wurtzite structure and its alloys with magnesium oxide (ZnMgO having a wurtzite structure) are being investigated. It is known to alloy zinc oxide with magnesium oxide to form high quality single crystal films having magnesium content between zero and 34 at. % while retaining the hexagonal zinc oxide lattice structure, as described, for example, in the publications entitled MgxZn1xe2x88x92xO as a II-VI Wide Gap Semiconductor Alloy to Ohtomo et al., Appl. Phys. Lett., Vol. 72, No. 19, May 11, 1998, pp. 2466-2468; Optical and Structural Properties of Epitaxial MgxZn1xe2x88x92xO Alloys to Sharma et al., Appl. Phys. Lett., Vol. 75, No. 21, Nov. 22, 1999, pp. 3327-3329; Refractive Indices and Absorption Coefficients of MgxZn1xe2x88x92xO Alloys to Teng et al., Appl. Phys. Lett., Vol. 76, No. 8, Feb. 21, 2000, pp. 979-981. The bandgap of this alloy was found to be variable with magnesium content with an upper limit around 4.19 eV. This alloy may produce a bright ultraviolet (UV) luminescence at room temperature that is excitonic in nature. Since the exciton binding energy in the ZnO system is higher (approximately 60 meV) than that in the III-nitride system (approximately 20 meV for GaN), tightly bound excitons may be responsible for higher brightness or luminescence efficiency. As with the III-nitrides and their alloys, ZnMgO alloys having an hexagonal wurtzite structure may only be grown via lattice-matching epitaxy on hexagonal substrates, such as 6H-silicon carbide (0001) and zinc oxide (0001), and may only be grown via domain-matching epitaxy on more practical hexagonal substrates such as hexagonal (0001) sapphire (xcex1-Al2O3). Epitaxial growth of ZnMgO alloys having an hexagonal wurtzite structure may be possible on (111) planes of cubic silicon. However, as noted above, this may not be a practical plane of cubic silicon. Thus, these hexagonal zinc oxide systems as well as III-nitrides may not be epitaxially integrated with silicon (100) of cubic symmetry. This may be an important consideration as microelectronic devices and integrated circuits are fabricated almost exclusively on silicon (100) substrates.
Zinc oxide layers having hexagonal structures are described, for example, in Optically Pumped Lasing of ZnO at Room Temperature to Bagnall et al., Appl. Phys. Lett., Vol. 70, No. 17, Apr. 28, 1997, pp. 2230-2232; Defects and Interfaces in Epitaxial ZnO/xcex1-Al2O3 and AlN/Zn O/xcex1-Al2O3 Heterostructures to Narayan et al., J. Appl. Phys., Vol. 84, No. 5, Sep. 1, 1998, pp. 2597-2601; Room-Temperature Ultraviolet Laser Emission from Self-Assembled ZnO Microcrystallite Thin Films, Appl. Phys. Lett., Vol. 72, No. 25, Jun. 22, 1998, pp. 3270-3272; Excitonic Structure and Absorption Coefficient Measurements of ZnO Single Crystal Epitaxial Films Deposited By Pulsed Laser Deposition, J. Appl. Phys., Vol. 85, No. 11, Jun. 1, 1999, pp. 7884-7887; and U.S. Pat. No. 6,046,464 to Schetzina. Epitaxial and polycrystalline zinc oxide films may provide lasing action. Zinc oxide films may also luminesce very brightly in UV and grow 2-D on c-plane sapphire with a low density of extended defects. Zinc oxide alloys having hexagonal wurtzite structure are described, for example, in U.S. Pat. No, 5,955,178 to Orita et al.
Embodiments of the present invention provide electronic devices having a cubic alloy layer comprising magnesium oxide and at least one of zinc oxide and cadmium oxide. These cubic alloy layers can be integrated with cubic (100) silicon substrates via domain-matching epitaxy where four lattice constants of the cubic alloy can match with three lattice constants of the silicon substrate. Since the critical thickness (i e., the minimum thickness to eliminate occurrence of misfit dislocations caused by the lattice mismatch) under this domain matching epitaxy may be less than one monolayer, many if not all of the misfit dislocations may be introduced at the beginning of the formation of the film, which may result in a film that is relaxed with many if not all of the dislocations confined to the interface. Cubic alloys according to embodiments of the present invention also may provide a cubic system having a broader bandgap than the conventional hexagonal wurtzite III-nitride system InNxe2x80x94GaNxe2x80x94AlN. Moreover, the excitons in zinc oxide may be more tightly bound than the excitons of the III-nitride system. As a result, optoelectronic devices having cubic zinc oxide alloy layers according to embodiments of the present invention may provide improved optoelectronic properties such as luminescence efficiency over conventional III-nitride systems. For example, cubic zinc oxide alloy layers of embodiments of the present invention may provide more efficient light emission for optoelectronic devices such as LEDs and lasers, as well as brighter excitonic light emission at room temperature.
More specifically, electronic devices according to embodiments of the present invention include a substrate and an alloy layer comprising magnesium oxide and at least one of zinc oxide and cadmium oxide on the substrate. The alloy has a cubic structure. In some embodiments, the alloy layer preferably consists essentially of between about 0.1 and 40 at. % zinc and between about 60 an 99.9 at. % magnesium. Alternatively, in other embodiments, the alloy layer preferably consists essentially of between about 0.1 and 15 at. % cadmium and between about 85 and 99.9 at. % zinc. The alloy layer may be a monocrystalline alloy layer. The substrate may consist of material having a cubic structure and may contain, for example, magnesium oxide, silicon, and/or gallium arsenide. The substrate may also consist of material having a hexagonal structure and may contain, for example, xcex1-Al2O3 sapphire and/or 6H-silicon carbide.
In some embodiments, the alloy layer may be a domain-matched epitaxial layer that is directly on the substrate. Alternatively, in other embodiments, the microelectronic device may include a buffer layer between the substrate and the alloy layer. The buffer layer may be a domain-matched epitaxial layer directly on the substrate, and the alloy layer may be a lattice-matched epitaxial layer directly on the buffer layer. In other embodiments, the electronic device may include a titanium nitride layer between the substrate and the alloy layer. The substrate may be silicon, and the titanium layer and the silicon substrate may form a titanium nitride-silicon epilayer hetereostructure. In other embodiments, the electronic device may include a magnesium oxide layer between the titanium nitride layer and the alloy layer. The magnesium oxide layer, the titanium nitride layer, and the silicon substrate may form a magnesium oxide-titanium nitride-silicon epilayer heterostructure.
In other embodiments, the alloy layer is a first alloy layer having a first bandgap preferably with electron doping. The electronic device includes a second alloy layer with a second bandgap narrower than the first bandgap on the first alloy layer and a third alloy layer having a third bandgap preferably with hole doping wider than the second bandgap on the second alloy layer to thereby provide a quantum well. The third alloy layer may comprise magnesium oxide and at least one of zinc oxide and cadmium oxide and have a cubic structure. In still other embodiments, the electronic device has a plurality of alternating layers of the first alloy and the second alloy on the substrate to provide a multiple quantum well.
Electronic devices, according to embodiments of the present invention, are fabricated by forming an alloy layer comprising magnesium oxide and at least one of zinc oxide and cadmium oxide and having a cubic structure on a substrate. In some embodiments, the forming step may include domain-matching epitaxially growing the alloy layer on the substrate. The forming step is preferably performed by pulsed laser depositing the alloy layer on the substrate. In other embodiments, a buffer layer may be formed on the substrate prior to the forming of the alloy layer, and the step of forming the alloy layer on the substrate may include forming an alloy layer comprising magnesium oxide and at least one of zinc oxide and cadmium oxide and having a cubic structure on the buffer layer on the substrate. In other embodiments, forming the buffer layer on the substrate may include the step of domain-matching epitaxially growing the buffer layer on the substrate, and forming the alloy layer on the buffer layer on the substrate may include the operation of lattice-matching epitaxially growing the alloy layer on the buffer layer. The substrate may be silicon, and the buffer layer may be titanium nitride. Accordingly, electronic devices having improved bandgap, increased binding energy of excitons, and/or reduced density of growth and/or misfit dislocations compared with conventional III-nitride electronic devices may be provided.